1. Field of the Invention
The present invention relates to a multipoint focus detecting apparatus for an optical instrument such as an SLR camera.
2. Description of the Prior Art
In recent years, SLR cameras provided with a multipoint focus detecting unit for determining a focus state (defocus) at each of a plurality of focus detection zones (focusing points) have been developed. In the optical system of a conventional multipoint focus detecting unit, other than the central focus detection zone arranged over the optical axis, one or more off-center focus detection zones are arranged away from the optical axis, and bundle of rays which form an object image which are passed through the central and off-center focus detection zones are respectively deflected by corresponding mirrors to be incident on corresponding light receiving elements arranged in a horizontal line. For instance, a light bundle of an object image which is passed through the horizontally-elongated central focus detection zone is deflected once by a mirror to be focused on a corresponding horizontally-elongated array of light receiving elements. At this time, the image observed through the horizontally-elongated central focus detection zone is divided into two, by a corresponding pair of separator lenses, to be formed as two separate images on the corresponding horizontally-elongated array of light receiving elements. At the same time, another light bundle of the object image which is passed through a vertically-elongated off-center focus detection zone is deflected twice by two mirrors to be focused on a corresponding horizontally-elongated array of light receiving elements. The image observed through the vertically-elongated off-center focus detection zone is divided into two by a corresponding pair of separator lenses to be formed as two separate images on the corresponding horizontally-elongated array of light receiving elements. The central and off-center pairs of separator lenses are arranged away from the central and off-center light receiving elements by a predetermined distance.
In the optical system of such a conventional multipoint focus detecting unit, the principal rays which have respectively passed through the corresponding pairs of separator lenses to proceed towards the corresponding arrays of light receiving elements, are not parallel to one another. Accordingly, once a mechanical dimensional error occurs and/or the arrays of light receiving elements deviate from the original positions along the optical axis direction thereof, the light bundles which are respectively passed through the corresponding pairs of separator lenses cannot be precisely led to the corresponding arrays of light receiving elements, which makes it difficult to adjust the position of each light bundle relative to the corresponding arrays of light receiving elements. Since the principal rays of the light bundles which are respectively passed through the corresponding pairs of separator lenses are not parallel to one another, the respective spaces among the principal rays vary if the arrays of light receiving elements deviate forwardly or rearwardly. This requires the position of each array of light receiving elements to be adjusted in the forward/reward direction thereof.